Sensor-independent oscillation amplitude control

ABSTRACT

A device is described for generating an electric voltage by which a body of a capacitive and/or inductive sensor capable of vibration, such as a capacitive micromechanical rotational rate sensor in particular, is induced to vibrate. In order to reduce the manufacturing cost of the sensor, a voltage generating device is provided which induces a constant mechanical deflection of the body capable of vibration, this deflection being independent of the manufacturing tolerances of the sensor.

FIELD OF THE INVENTION

The present invention relates to a device for generating an electricvoltage.

BACKGROUND INFORMATION

A known rotational rate sensor produced by microsystem technology has anoscillating weight which oscillates about its axis of rotation. Theoscillating weight has a comb structure, i.e., it is formed by a combstructure which alternately meshes with a first stationary combstructure and with a second stationary comb structure of the sensor asit oscillates. This arrangement forms two capacitors whose capacitanceschange in opposite directions over time. If the rotational rate sensorexperiences a rotational rate perpendicular to the axis of torsionalvibration of the oscillating weight, one side of the oscillating weightmoves toward the substrate of the rotational rate sensor and the otherside moves away from it. These changes in distance are measuredcapacitively by electrically conducting surfaces beneath the oscillatingweight. The comb structures which are stationary with respect to thesensor and the comb structure which is provided on the oscillatingweight are acted upon by an alternating voltage, thereby inducingoscillation of the oscillating weight.

To obtain a high signal-to-noise ratio of the test signal whichrepresents the rotational rate, the deflection of the moving structureof the sensor must be maximized.

In the case of a known capacitive micromechanical sensor, such as arotational rate sensor manufactured by planar silicon processes inparticular, the change in capacitance depends not only on the deflectionof the moving structure but also on the gap distance. Gap distance isunderstood to refer to the average distance between the “teeth” of themovable comb structure and the two stationary comb structures in thecase of a stationary oscillating weight. Since the gap distance may varyfrom one sensor to the next due to the manufacturing technology, eachsensor must be adjusted individually to achieve maximum deflection, i.e,maximum vibration amplitude of the movable structure. Not only is thiscomplicated, but it may also result in the movable structure strikingagainst the stationary structure, which could damage the sensor.

SUMMARY OF THE INVENTION

The device according to the present invention has the advantage over therelated art in particular that, regardless of the manufacturingtolerances, it automatically adjusts a predefined deflection of theoscillating weight of a capacitive or inductive sensor. This eliminatesindividual manual adjustment of each sensor for setting a virtuallymaximum deflection of the oscillating weight in order to obtain amaximum signal-to-noise ratio. This makes it possible to manufacturecapacitive and inductive sensors such as rotational rate sensors inparticular less expensively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the first part of a schematic diagram of a first embodimentof a sensor-independent vibration amplitude regulating device accordingto the present invention.

FIG. 2 shows the second part of the schematic diagram of the firstembodiment of a sensor-independent vibration amplitude regulating deviceaccording to the present invention.

FIG. 3 shows the first part of the schematic diagram of the secondembodiment of a sensor-independent vibration amplitude regulating deviceaccording to the present invention.

FIG. 4 shows the second part of the schematic diagram of the secondembodiment of a sensor-independent vibration amplitude regulating deviceaccording to the present invention.

DETAILED DESCRIPTION

For the sake of simplicity, the schematic diagram of asensor-independent vibration amplitude regulating device according tothe present invention has been divided into FIGS. 1 and 2 plus 3 and 4.An output of a first part of the schematic diagram, labeled as A inFIGS. 1 and 3, is connected electrically to an input of a second part ofthe schematic diagram, labeled as E in FIGS. 2 and 4.

First part 100 of the schematic diagram of the first embodiment of thevibration amplitude regulating device according to the presentinvention, as illustrated in FIG. 1, shows at the left a schematicdiagram 101 of another comb structure arrangement having a combstructure movable with the oscillating weight and two stationary combstructures of the type described above. These additional comb structuresare used to sense the deflection of the oscillating weight. Diagram 101shows two capacitors 102 and 103, which are formed by. the two combstructures, these comb structures being stationary with respect to thesensor and having the function of sensing the deflection, and by themovable comb structure oscillating between the two former combstructures.

Furthermore, first part 100 of the schematic diagram shows a firstsignal path 107, a second signal path 108, an adder 110, a demodulator111, an amplifier 121 and a common-mode regulating apparatus 109.

First signal path 107 has a terminal 104, a C/U converter 112 and anamplifier 113. Terminal 104 is connected to the input of C/U converter112, whose output is connected to the input of amplifier 113, and theoutput of amplifier 113 is connected to a first input of adder 110. Inan identical manner, second signal path 108 has a terminal 106, a C/Uconverter 114 and an amplifier 115. Terminal 106 is connected to theinput of C/U converter 114 whose output is connected to the input ofamplifier 115, and the input of amplifier 115 is connected to a secondinput of adder 110. The output of adder 10 is connected to a first inputof demodulator 111 and its output is connected to third input ofamplifier 121.

C/U converters 112 and 114 are preferably optical amplifiers wired asinverting amplifiers having on-chip capacitance C_(RK) in the feedback;these are charge amplifiers.

Common-mode regulating apparatus 109 (CMRA) has an adder 120, aregulator 119, preferably an I regulator, a modulator 118, a capacitor116 having a capacitance C_(I) and a capacitor 117 also havingcapacitance C_(I). A first input of adder 120 is connected to the outputof C/U converter 112, i.e., the input of amplifier 113, and a secondinput of adder 120 is connected to the output of C/U converter 114,i.e., the input of amplifier 115. The only output of adder 120 isconnected to the input of regulator 119, and the output of regulator 119is connected to both the input of modulator 118 and to a regulatingterminal of amplifier 12l.

The output of modulator 118 is connected to a first terminal ofcapacitor 116 and to a first terminal of capacitor 117. The secondterminal of capacitor 116 is connected to the input of C/U converter112, i.e., terminal 104, and the second terminal of capacitor 117 isconnected to the input of C/U converter 114, i.e., terminal 106.

The second part of the schematic diagram of the first embodiment of thevibration amplitude regulating device of a rotational rate sensor, asshown in FIG. 2, shows input E connected to output A shown in FIG. 1, aphase quadrature device 201, an output stage 203, a terminal 204, aterminal 205, an adder 208, an amplifier 209, a rectifier 206 and aregulator 207, where regulator 207 forms part of an automatic gaincontrol (AGC).

Input E of the second part of the schematic diagram of the vibrationamplitude regulating device of a rotational rate sensor shown in FIG. 2is connected to the input of the phase quadrature device 201, the outputof phase quadrature device 201 being connected to the input of amplifier202, the output of amplifier 202 being connected to an input of outputstage 203, and one output of output stage 203 being connected toterminal 204 and another output of output stage 203 being connected toterminal 205. The input of phase quadrature device 201 is also connectedelectrically to the input of rectifier 206, whose output is connected tothe first input of adder 208, whose output is in turn connected to theinput of regulator 207, and finally, the output of regulator 207 isconnected to an additional input of output stage 203. The second inputof adder 208 is connected to the output of amplifier 209.

A setpoint voltage U_(setpoint) is applied to the input of amplifier 209and sets the desired maximum deflection of the oscillating weight forall sensors of the same type.

The function of the vibration amplitude regulation of a rotational ratesensor according to the present invention is described in detail below.It is assumed that the oscillating weight oscillates about its restingposition.

The time-dependent capacitance (C(t)) of capacitor 102 or capacitor 103for identical capacitors, i.e., comb structures, is described in firstapproximation as:

C ₁₀₂(t)=n∈*(((1₀+δ1(t)))*h)/d=C ₀ +δC(t)  (1)

C ₁₀₃(t)=n*∈*(((1₀+δ1(t)))*h)/d

=C₀ −δC(t)  (2)

where:

1 ₀: basic overlapping of the movable comb structure with thecorresponding stationary comb structure;

δ1: deflection of the movable comb structure;

h: height of the movable comb structure;

d: gap distance of the movable comb structure from the stationary combstructure, i.e., the distance (ideally always identical) betweenadjacent “teeth” or fingers of movable and stationary comb structures;

n: number of overlapping fingers of movable and stationary combstructures;

∈: dielectric constant of the medium, air in particular, between themovable and the stationary comb structures;

δC: time-dependent change in capacitance as a function of the deflectionof the movable comb structure relative to the stationary comb structure;

C₀: resting capacitance, i.e., the capacitance of the capacitor formedby the movable comb structure and the stationary comb structure when themovable comb structure is stationary.

It holds that:

δC/C ₀=δ1/1₀  (3)

i.e., the relative change in capacitance due to deflection of themovable comb structure is equal to δ1/1 ₀. The movable comb structure isacted upon by an alternating voltage U_(HF) from a device (not shown) atfrequency f_(HF) via terminal 105. Frequency f_(HF) of alternatingvoltage U_(HF) is much higher than operating frequency f_(sensor)supplied to the sensor via the driving comb structures. For example,frequency f_(HF) of alternating voltage U_(HF) corresponds approximatelyto 16 times operating frequency f_(sensor), operating frequencyf_(sensor) amounting to approx. 1.5 kHz, for example. It is self-evidentthat this information applies only to examples of one specific sensor.

An alternating voltage having a frequency f_(HF) is applied to terminals104 and 106, frequency f_(HF) being amplitude-modulated with theoperating frequency of sensor f_(sensor).

The time-dependent capacitance of first capacitor 102 is converted byC/U converter 112 into a corresponding electric voltage, amplified byamplifier 113 and sent to adder 110. The capacitance of second capacitor103 showing an inverse time dependence in comparison with thecapacitance of the first capacitor is converted by C/U converter 114into a corresponding electric voltage, amplified by amplifier 115 andalso sent to adder 110.

The alternating voltage delivered by adder 110 is sent to demodulator111. Demodulator 111 demodulates, i.e., multiplies the alternatingvoltage delivered by adder 110 by the sign of alternating voltageU_(HF).

Adder 110 forms the difference between the electric signals in firstsignal path 107 and second signal path 108, amplified by gain factor gby amplifier 113 and amplifier 115; therefore, the alternating voltagedelivered by demodulator 111 at its output is:

U_(FE)=2*g*δC/C _(RK) *U _(HF)=2* g*δ1/1₀ *C ₀ /C _(RK) *U _(HF)  (4)

where:

g: gain factor;

C_(RK): feedback capacitance of C/U converter 112 and identical C/Uconverter 114;

U_(HF): alternating voltage U_(HF);

U_(FE): the alternating voltage delivered by demodulator 111 afterdemodulation, i.e., multiplication by sign U_(HF),

this means that, due to the differentiation of the electric signals atthe output of first signal path 107 and second signal path 108 performedby adder 110, the common-mode component caused by resting capacitance C₀is eliminated.

An essential aspect of the present invention is providing measures sothat U_(FE) is independent of the resting capacitance C₀ of the sensor,which is subject to certain fluctuations due to manufacturingtolerances.

According to a preferred embodiment of the present invention, bothelectric voltage U_(LV1) between the output of C/U converter 112 andamplifier 113 and electric voltage U_(LV2) between the output of C/Uconverter 114 and amplifier 115 are picked up, electric voltage U_(LV1)being sent to the first input of adder 120 and electric voltage U_(LV2)being sent to the second input of adder 120.

The electric voltage delivered by C/U converters 112 and 114 at theiroutputs is:

U _(LV1,LV2)=(C ₀ +/−δC)/C _(RK) +U _(HF)  (5)

The result of addition of the electric voltages performed by adder 120is an output voltage U_(add) of adder 120, for which it holds that:

U _(add) =f((C ₀ +δC)+(C ₀ −δC))=f(C ₀)  (6)

i.e., the output voltage of adder 120 is a function of restingcapacitance C₀.

Output voltage U_(add) of adder 120 is sent to regulator 119, preferablyan I regulator delivering an output voltage U_(I) which is sent to aninput of modulator 118 and also to the regulating terminal of anplifier121.

Modulator 118 also receives alternating voltage U_(HF), and the outputsignal delivered by modulator 118 goes to a first terminal of eachcapacitor 116 and 117, both having a capacitance C_(I). The secondterminal of capacitor 116 is connected to the input of C/U converter 112in signal path 107, and the second terminal of capacitor 117 isconnected to the input of C/U converter 114 in signal path 108.

Capacitors 116 and 117 receive a voltage via regulator 119 such that theoutput signal of adder 120 has an amplitude of approx. 0 volt, i.e.,capacitors 116 and 117 almost completely compensate resting capacitanceC₀ of the respective sensors.

Common-mode regulating apparatus 109 (CMRA) therefore responds only tocommon-mode signals, i.e., direct voltage signals, at the input end. Theoutput of regulator 119 changes its voltage in regulating operationuntil there is no longer a common mode signal at the input of adder 120.This condition is met when the following holds:

U _(HF) *C ₀ =−U _(I) *C _(I)  (7)

i.e., U _(I) =−C ₀ /C _(I) *U _(HF)  (8)

i.e., voltage U₁, is directly proportional to resting capacitance C₀.

Amplifier 121 performs an amplification g_(var) of voltage U_(FE) as afunction of the particular resting capacitance via voltage U_(I) appliedto amplifier 121, for which the following equation holds:

g _(var) =C _(I) /C ₀  (9)

For electric voltage U delivered at the output of amplifier 121, thisyields:

 U=2*g*δ1/1₀ *C _(I) /C _(RK) *U _(HF)  (10)

where δC/C₀=δ1/1 ₀ (see equation (3)),

i.e., the voltage applied at the output of amplifier 121, i.e., atoutput A, is independent of resting capacitance C₀ of the particularsensor whose vibrational amplitude is to be regulated. Voltage U andthus change δ1 in the path of the movable sensor element depend only onlow-tolerance voltage U_(HF), which is determined by the electronicregulation and/or measurement devices, and basic overlap 1 ₀. Basicoverlap 1 ₀ is settable with a high precision, however, in particular inthe case of a micromechanical sensor manufactured from semiconductorlayers by using planar silicon processes.

Voltage U delivered by amplifier 121 is sent to phase quadrature device201, which sends voltage U, 90° out of phase, to the input of amplifier202 and sends amplified out-of-phase voltage U to an input of outputstage 203.

Furthermore, voltage U delivered by amplifier 121 is sent to the inputof rectifier 206 via input E, i.e., the input of the phase quadraturedevice. Setpoint voltage U_(setpoint) amplified by amplifier 209 issubtracted by adder 208 from voltage U rectified by rectifier 206, andthe output signal of adder 208 is sent to the input of regulator 207.Regulator 207 changes the voltage at its output until its input voltageis virtually zero. Regulator 207, preferably a PI regulator and/or anautomatic gain control regulator (AGC) controls output stage 203 so thatthe output stage delivers a voltage to the drive comb structures of thesensor (not shown) via terminals 204 and 205, so that the vibrationalamplitude of the oscillating sensor element, i.e., the oscillatingweight, is constant and virtually at a maximum.

The second embodiment of the vibration amplitude regulating deviceaccording to the present invention as illustrated in FIGS. 3 and 4differs from the first embodiment illustrated in FIGS. 1 and 2 in thatinstead of setpoint voltage U_(setpoint) voltage U_(I) delivered at theoutput of regulator 119 is applied to the second input of adder 208;furthermore, voltage U_(I) is not applied to amplifier 121 in the secondembodiment, so the amplifier implements a constant gain g_(const). Thefollowing thus holds for the output voltage of amplifier 121:

 U=2*g*δ1/1₀ *C ₀ /C _(RK) *U _(RF) *g _(const)  (11)

The regulator, i.e., AGC regulator 207 changes its output voltage untiloutput voltage U of amplifier 121 corresponds to AGC reference inputvariable U_(I) (or a variable proportional thereto). As in the firstembodiment, this also means that the amplitude of vibration of theoscillating sensor element, i.e., the oscillating weight, is independentof resting capacitance C₀, which is subject to manufacturing tolerances.

Gap distance manufacturing tolerances due to overetching now no longerhave any effect on the deflection and thus the speed of the movablesensor element. A more complex and thus more expensive adjustment ofeach finished sensor to adjust the desired deflection is no longernecessary when using the sensor-independent vibrational amplituderegulating device according to the present invention.

As explained above, the sensor-independent vibration amplituderegulating device according to the present invention regulates thevibration amplitude of the oscillating weight of a capacitive sensorsuch as a rotational rate sensor in particular. It is self-evident thatthe vibrational amplitude regulating device described here may also beused in a modified form to regulate the amplitude of vibration of theoscillating weight of an inductive sensor, e.g., such as a rotationalrate sensor in particular. Such a modified form of the vibrationamplitude regulating device according to the present invention takesinto account in particular the fact that instead of capacitances, thereare inductances which are subject to manufacturing tolerances, in aninductive sensor.

LIST REFERENCE NOTATION

100 first part of the schematic diagram of the vibration amplituderegulating device according to the present invention

101 schematic diagram of the comb structures of a capacitive rotationalrate sensor for sensing the deflection of its oscillating weight

102 capacitor

103 capacitor

104 terminal

105 terminal

106 terminal

107 first signal path

108 second signal path

109 common-mode regulating apparatus (CMRA)

110 adder

111 demodulator

112 C/U converter

113 amplifier

114 C/U converter

115 amplifier

116 capacitor

117 capacitor

118 modulator

119 regulator

120 adder

121 amplifier

200 second part of the schematic diagram of the vibration amplituderegulating device according to the present invention

201 phase quadrature device

202 amplifier

203 output stage

204 terminal

205 terminal

206 rectifier

207 regulator

208 adder

209 amplifier

What is claimed is:
 1. A device for generating an electric voltagewhereby a vibration of a body of at least one of a capacitive sensor andan inductive sensor capable of vibration is induced, comprising: avoltage generating device that generates an electric voltage that isproportional to at least one of a resting capacitance and an inductionof a magnetic field of the at least one of the capacitive sensor and theinductive sensor.
 2. The device as recited in claim 1, wherein: the atleast one of the capacitive sensor and the inductive sensor includes acapacitive micromechanical rotational rate sensor.
 3. The device asrecited in claim 1, wherein: the voltage generating device forms part ofa regulating circuit for regulating an amplitude of the vibration of thebody.
 4. The device as recited in claim 3, wherein: the voltagegenerating device includes a common-mode regulating apparatus thatresponds only to a common-mode signal at an input end.
 5. The device asrecited in claim 4, wherein at least one of: the common-mode regulatingapparatus includes a first adder and a regulator, and an output of theregulator changes a voltage thereof in regulating operation untilvirtually no common-mode signal is applied at an input of the regulator.6. The device as recited in claim 5, wherein: the regulator includes anI regulator.
 7. The device as recited in claim 5, wherein: the at leastone of the capacitive sensor and the inductive sensor includes twoelements whose capacitance is variable over time in opposite directions,the two elements being formed in part by the body capable of vibration,and a change in capacitance of the two elements being detectedseparately in a first signal path and a second signal path.
 8. Thedevice as recited in claim 7, wherein: an output signal of the firstadder is sent to the input of the regulator, a first input of the firstadder picks up a first signal in the first signal path, and a secondinput of the first adder picks up a second signal in the second signalpath.
 9. The device as recited in claim 8, further comprising: amodulator, wherein at least one of: an output signal of the regulator issent to an input of the modulator, the modulator modulating the outputsignal of the regulator in accordance with a frequency of a voltagesupplied to the two elements. an output signal of the modulator is sentto a first terminal of a first capacitor having a first capacitance andto a first terminal of a second capacitor having a second capacitance, asecond terminal of the first capacitor is electrically connected to thefirst signal path, and a second terminal of the second capacitor iselectrically connected to the second signal path.
 10. The device asrecited in claim 9, further comprising: an amplifier including a firstinput to which the output signal of the regulator is sent and foramplifying a voltage applied thereto by a factor that depends on the atleast one of the capacitive sensor and the inductive sensor and that isone of proportional to and equal to a quotient of the first capacitanceand the resting capacitance.
 11. The device as recited in claim 10,further comprising: a second adder, and a demodulator, wherein: anoutput signal of the first signal path is sent to a first input of thesecond adder, an output signal of the second signal path is sent to asecond input of the second adder, and an output signal of the secondadder is demodulated by a demodulator and sent to the amplifier foramplification.
 12. The device as recited in claim 11, wherein: thedemodulator performs a demodulation using the frequency of the voltagesupplied to the two elements.
 13. The device as recited in claim 12,wherein: an output voltage of the amplifier is kept constant.
 14. Thedevice as recited in claim 13, wherein: the output voltage of theamplifier is proportional to the resting capacitance of the at least oneof the capacitive sensor and the inductive sensor, and an electricvoltage is a reference variable of the regulating circuit for regulatingthe amplitude of the vibration of the body capable of vibration.